1. Field of the Invention
This invention relates to sensing movement or position between two members by means of signals induced by magnetic fields. More particularly, this invention is directed to an induced current position transducer that compensates certain potential signal errors for a read head that is not perfectly parallel to a scale member.
2. Description of Related Art
Various movement or position transducers are currently available, such as optical, capacitive, magnetic and inductive transducers. These transducers often involve placing a transmitter and a receiver in various geometric configurations to measure movement between two members of the transducer.
Optical, capacitive, and magnetic transducers, are sensitive to contamination. Therefore, using such transducers in most manufacturing or shop environments is impractical. Using such transducers in a shop environment requires expensive and sometimes unreliable environmental seals or other methods of encapsulating the transducer to keep dust, oils and ferromagnetic particles from contaminating the transducer.
Pending U.S. patent application Ser. No. 08/441,769, filed May 16, 1995, and herein incorporated by reference in its entirety, describes an induced current position transducer usable in high accuracy applications. Pending U.S. patent application Ser. Nos. 08/645,483 and 08/645,490, both filed May 13, 1996, and both herein incorporated by reference, describe incremental position inductive calipers and linear scales, including signal generating and processing circuits. Pending U.S. patent application Ser. No. 08/788,469, now U.S. Pat. No. 5,886,519, Ser. No. 08/790,494, now U.S. Pat. No. 5,841,274, and Ser. No. 08/790,459, now U.S. Pat. No. 5,894,678, all filed on Jan. 29, 1997, each herein incorporated by reference, describe absolute position inductive calipers and electronic tape measures using this induced current transducer.
This induced current position transducer employs two members that are movable relative to each other. A read head contains an active transmitter for generating a changing magnetic field and a passive receiver for receiving and sensing the field and producing a receiver output signal. A scale includes a plurality of flux modulators. The flux modulators modulate the magnetic field, and thus the induced current in the receiver, depending on the position of the scale relative to the read head.
The transmitter includes a field source that produces a changing magnetic flux through a flux region. The receiver includes a receiver winding arranged in a prescribed pattern of flux-receiving areas along a measuring axis and within the flux region. The receiver winding passively generates a signal in response to the sensed changing magnetic flux. The amplitude and polarity of this signal is a function of the position of the read head relative to the scale. The receiver winding is formed by crossing a conductive element over itself at predetermined intervals to approximate a sinusoidal wave that reverses at one end. In this manner, the receiver winding has adjacent loops having alternating clockwise and counter-clockwise orientations. The magnetic field generated by the transmitter that passes through these loops with alternating orientations will generate EMFs having alternating polarities.
Alternatively, with suitable signal processing such as that detailed in the other U.S. patent applications incorporated herein, the transmitter and receiver/sensor functions may be switched. That is, the transmitter winding may be formed by crossing a conductive element over itself at predetermined intervals to have adjacent loops with alternating clockwise and counter-clockwise orientations. The magnetic fields generated by each of these loops will have alternating polarities. The receiver in this case is a simple loop.
The scale includes a plurality of flux modulators that are regularly positioned at a predetermined interval along the scale. As the scale is moved along the read head, the flux modulators move within the flux region to modulate the magnetic flux proximate to those flux modulators. The flux modulators within the flux region vary the induced current and thus the signal generated by the receiver winding will be a function of the relative position between the read head and the scale.
The induced current transducer, and the calipers, linear scales and electronic tape measures disclosed in these pending U.S. patent applications are readily manufactured using known printed circuit board technology. This transducer system is generally immune to contamination by particles, including ferromagnetic particles, oil, water, and other fluids.
FIG. 1 shows an induced current position transducer 100 having a read head 110 and a scale 120. The read head 110 includes a transmitter winding 112 and at least one receiver winding 114. The receiver winding 114 defines a plurality of positive polarity loops 116 and a plurality of negative polarity loops 118. The positive and negative polarity loops 116 and 118 extend along a measuring axis 130 of the read head 110 within the transmitter winding with a repetition rate corresponding to the scale wavelength .lambda..
The scale 120 includes a plurality of flux modulators 122. The flux modulators 122 can be either flux disrupters or flux enhancers. The flux modulators 122 are positioned at a pitch equal to the wavelength .lambda.. Each flux modulator 122 extends along the measuring axis 130 approximately one-half of the wavelength .lambda. or less.
The flux disrupters are formed by a thin layer of conductive material. The changing magnetic field generated by the transmitter winding 112 generates eddy currents in the thin conductive layer. The eddy currents generate magnetic fields having a direction opposite that of the magnetic field generated by the transmitter winding 112. This reduces, or disrupts, the net magnetic field flux in the regions adjacent to the thin conductive layer flux disrupters.
The flux enhancers are formed by portions of a high magnetic permeability material. The magnetic field flux generated by the transmitter winding 112 preferentially passes through the high magnetic permeability flux enhancers. Thus, the magnetic flux within the flux enhancers is relatively denser, while the magnetic flux in areas laterally adjacent to the flux enhancers is relatively less dense.
In either case, the flux modulators spatially modulate the magnetic flux generated by the transmitter winding and the effective flux coupling between the transmitter and the receiver. It should be appreciated that the flux modulators 122 can indicate an alternating arrangement of flux disrupters or flux enhancers. It should also be appreciated that any of the read head layouts and scale layouts disclosed in the incorporated U.S. Patent Applications can be used with the pitch compensation technique of this invention.
When the read head 110 is positioned over, approximately parallel to, and in proximity to the scale 120, the magnetic field generated by the transmitter winding 112 is modulated by the flux modulators 122. The modulated magnetic field induces a position dependent net EMF in the receiver winding 114.
As shown in FIG. 2, a second induced current transducer 100' does not have a transmitter winding that surrounds the receiver winding. Instead, this second induced current transducer has two transmitter windings 112A and 112B symmetrically placed outside of the receiver winding 114 and connected in series in such a way that the direct field from the transmitter windings 112A and 112B to the receiver winding 114 is minimized. On the scale 120 the flux modulators are replaced with loops 150 that transfer the field from the transmitter area to the area under the receivers.
The current in the upper transmitter loop 112A flows clockwise, causing an induced current in the underlying scale loops 152 that flows counterclockwise. The current in the lower transmitter loop 112B flows counterclockwise, inducing a current that flows clockwise in the underlying scale loops 154. Thus, the alternating loops on the scale carry currents of alternating direction. An example of this second induced current transducer is described in U.S. patent application Ser. No. 08/834,432, filed Apr. 16, 1997, and herein incorporated by reference in its entirety.
In a manner similar to the transducer of FIG. 1, the transmitter and receiver/sensor functions may be switched. In this case, the transmitter may comprise the winding 114 that has alternating polarity loops while the receiver may comprise the winding 112.
When the read head 110' is moved over the scale 120' along the measurement direction, the EMF induced in the receiver winding 114 will alternate as a periodic function of the relative position between the two members with a wavelength equal to the distance between equal polarity loops on the scale. This second induced current transducer 100' is driven with the same kind of signal and the receiver signals are processed in the same way as in the first induced current transducer shown in FIG. 1. Thus, the pitch compensation principles in this invention are equally applicable to the first and second induced current transducers 100 and 100'.
These transducers and other similar types of inductive transducers suffer from undesired errors when the active read head is misaligned relative to the passive scale. One aspect of this misalignment is "pitch". As shown in FIG. 3, in pitch, the read head 110 is rotated from a position parallel to the scale 120 about a pitch axis 132. The pitch axis 132 lies in the plane of the read head 110 and is substantially perpendicular to a measuring axis of the transducer. Thus, one end of the read head will be closer to the scale than the other end of the read head.
The transducers of FIGS. 1 and 2 are shown with single receiver windings 118 and 114, respectively, for simplicity of description. However, practical devices more commonly have multiple receiver windings displaced from each other along the measuring axis, such as windings placed in quadrature, for detecting direction of motion and for other practical reasons. If the read head of one of these induced current transducers is pitched relative to the scale elements, two of the major types of signal errors that can occur are: 1) a net unbalance in the induced amplitudes in the positive and negative loops within a single receiver winding, and 2) a net unbalance in the output signal amplitudes between the multiple receiver windings of the transducers. The unbalanced output of the positive and negative loops within a single receiver winding creates an undesirable position insensitive "DC" offset within the receiver winding signal, and additionally creates an undesirable +/- modulation unbalance within the receiver winding. The different output between multiple receiver windings appears as an undesirable modulation amplitude mismatch between those receiver windings, and additionally, if the individual windings exhibit DC offset, as an undesirable DC offset mismatch between the receiver windings. It should be appreciated that the +/- modulation unbalance and the DC offset mismatch are generally significantly smaller in magnitude than the DC offset and the modulation amplitude mismatch, respectively.
When a conductive scale support material is used, as in a stainless steel caliper beam, the strength of the transmitted field is attenuated compared to when a non-conductive scale base is used. The strength of the transmitted field is attenuated because the transmission field sets up eddy currents in the conductive scale support that in turn induces a counteracting field that reduces the strength of the transmission field. This effect reduces the strength of the response in the vicinity of each portion of the read head receivers to an extent that is dependent upon the gap between the read head and scale. Thus, this effect can distort the desired read head signal relationships when the read head is pitched relative to a conductive support.
The cross-talk from the transmission winding to a receiver winding is defined as the signal at the receiver winding terminal that is caused by the current directly induced by the transmitter field in the receiver windings independent of the flux modulators. In the first induced current transducer shown in FIG. 1, each sub-loop in the receiver windings is subject to a fairly strong directly induced current from the transmitter field. However, each receiver winding includes sub-loops that are wound in alternating directions that cause the induced currents to have alternating polarities. The resulting signals at the terminals of the receiver windings due to cross talk are thus nominally zero, because the number and size of positive and negative loops in each receiver winding are equal. However, balance between the positive and negative currents induced directly by the transmitter winding may be disturbed by an increase in the cross-talk caused by a pitch misalignment of the read head relative to a conductive plane. FIG. 3 illustrates why this happens.
In FIG. 3, read head 110 is positioned over a conductive plane 123, such as a conductive beam supporting the scale 120. Three sets of positive and negative loop pairs 1-3 of the receiver winding are positioned along the measuring axis. Because of the counteracting field generated by the eddy currents in the conductive base, the resulting field in the plane of the winding varies with the distance between the read head 110 and the scale 120. A pitch misalignment of the read head 110 induces an increase in the direct cross-talk between the transmitter winding and the receiver winding because one of the positive or negative loops will be, on average, closer to the conductive base than the other one of the positive and negative loops. In FIG. 3, the negative loops are, on average, closer to the conductive base than the positive loops. Therefore, an imbalance is created because a smaller net transmitter flux passes through the negative loops than through the positive loops. This imbalance shows up as a DC offset in the receiver signal.
If two receiver windings are present and offset from each other along the measuring axis as shown in FIG. 4, an additional problem occurs in that the cross-talk offset will be different in each receiver winding, due to their differing average gap distances. That is, there will be a DC offset mismatch between the two receiver windings. Also, the modulation amplitude in the two receiver windings will be different because of the differing average gap distance.
In the second induced current transducer shown in FIG. 2, the problem with the crosstalk offset is smaller, because the transmitter does not surround the receiver. Furthermore, the direct transmitter field through the receiver windings is nominally balanced by the interaction of the two transmitter windings. However, if this balance is not perfect, or disturbed by a roll misalignment of the read head relative to the scale, a cross-talk offset is created. Roll is a rotation around an axis parallel to the measurement axis.
As described in the incorporated application Ser. Nos. 08/441,769; 08/645,483 and 08/645,490, at least two receiver windings are usually required to unambiguously determine position and movement direction. When multiple receiver windings are present in the first and second transducers, the position is calculated from the relation of signals from these windings.
If the receiver windings are identical but offset from each other in the measurement direction, pitch misalignment of the read head relative to the scale of the transducer causes a modulation amplitude mismatch between the output signals of the two receiver windings. FIG. 4 shows the relative positions of two receiver windings 114A and 114B within the first and second induced current transducers. The two receiver windings 114A and 114B lie in the same plane and are offset one quarter of a wavelength from each other to generate quadrature output signals. As shown in FIG. 4, a pitch misalignment brings the first receiver winding 114A closer to the scale 120 than the second receiver winding 114B. As a result, a modulation amplitude mismatch (signal amplitude mismatch) is created because the signal from the first receiver winding 114A is stronger than that of the second receiver winding 114B. This amplitude mismatch induces a measurement error. This problem exists equally in multiple winding versions of the first and second induced current transducers shown in FIGS. 1 and 2.
FIG. 3 also illustrates how a pitch misalignment generates +/- modulation unbalance in a receiver winding of the first induced current transducer 100 and the second induced current transducer 100'. In other words, within a single winding, the signal modulation of one loop polarity by the scale is larger than the signal modulation of the other loop polarity by the scale. In the first and second induced current transducers 100 and 100', the read heads 110 and 110' and the scales 120 and 120', respectively are preferably positioned perfectly parallel to each other. However, due to manufacturing errors and/or tolerances, and as various components of the first and second induced current transducers 100 and 100' wear, the read heads 110 and 110' and the scales 120 and 120' may become relatively misaligned, as shown in FIG. 3. In particular, the read heads 110 and 110' and scale 120 become pitched by an angle .theta. about a pitch axis 132 from a perfectly parallel orientation indicated by a plane 134.
As shown in FIG. 3, as a result of the pitch, the "3-" negative polarity loop is positioned closest to the scale 120, while the "1+" positive polarity loop is positioned farthest away from the scale 120. Because different EMFs are induced in the receiver loops that are positioned farther away from the scale 120 than those that are closer, the magnitude of the EMF generated in the "1+" positive polarity loop is different than the magnitude of the EMF generated in the "1-" negative polarity loop. Similarly, the magnitude of the EMF generated in the "2-" negative polarity loop is different from the magnitude of the EMF generated in the "2+" positive polarity loop. This is true for each pair of adjacent loops across the read heads 110 and 110' along the measurement axis 130.
When FIG. 3 represents the first induced current transducer 100, the scale elements are flux modulators 122 and the modulation of the induced EMF in each loop will increase as the distance from the scale 120 decreases. Thus, the modulation of the EMF generated in the "1+" loop is less than the modulation of the EMF generated in the "1-" loop, and so on. The net effect is to increase the net modulation signal amplitude for the loop polarity that is on average closer to the scale due to pitch, which defines a +/- modulation unbalance due to the pitch.
When FIG. 3 represents the second induced current transducer 100', the scale elements represent the coupling loops 150. In this case, the inductive coupling provided by the coupling loops 150 between the transmitter winding and a particular loop of the receiver winding will be weaker as the distance from the scale member 120 to any portion of the read head 110' increases. The net effect is again to increase the modulation for the loop polarity that is on average closer to the scale due to pitch, which defines a +/- modulation unbalance due to the pitch. Thus, the read head winding 114 of the induced current transducer 100' shown in FIG. 2 will, in general, be pitch sensitive. It should be noted that the dual-polarity scale 120 shown in FIG. 2 overcomes this pitch sensitivity by theoretically eliminating DC components in the scale field which might interact with the pitch sensitivity of the read head winding 114. However, in general, other scales may be used in which there may exist a significant DC component in the scale field which introduces a DC component in the output signal from the winding 114 when the read head has a pitch misalignment relative to the scale. This creates an error in the output position signal when the processing electronics assumes no unbalance in the winding output signal.
Therefore, an induced current position transducer is needed that is accurate, inexpensive to manufacture and immune to contamination, and can produce substantially balanced signal components that are not adversely affected by pitch misalignment.